It is known to provide a communications network in which a group of electronic devices communicate with one another via a communications bus in the form of a plurality of electrical conductors of a cable. Each device may be referred to as a node of the network.
It is to be understood that maintenance of the integrity of the transmitted signal and minimising signal reflection and distortion due to high frequency wideband switching and wireless communications interference are important considerations in the engineering of transmission bus networks. It is therefore important to terminate a network with a terminating circuit having an appropriate matching impedance. If a circuit is incorrectly terminated, malfunction of the network can result, due for example to reflection at a node of signals propagating through the network and introduction of noise.
Networks are normally terminated at both ends, i.e. at a host device at one end of a communications cable and at a node at the opposite end of the cable.
GB2407237 discloses an electronic circuit for automatic termination of a plurality of conductors of a communications network. Such a circuit may be referred to as an auto-termination circuit.
The circuit has detecting means operable to detect current in at least one of the plurality of conductors and switching means operable to switch the circuit between being a continuing circuit, upon the detecting means detecting a current greater than a first predetermined threshold, and being a terminating circuit, upon the detecting means detecting current at, or less than, a second predetermined threshold wherein the detecting means comprises a sensing resistor connected in series with the at least one of a plurality of conductors, and means for detecting voltage across the sensing resistor. The document discloses using detecting means in the form of a differential amplifier connected across the sensing resistor.
A known problem with this method based on a sensing resistor is that the circuit is highly susceptible to noise irrespective of any online filtering. Such noise may be generated from mobile communication devices such as handheld radio communications devices operating within the frequency range 150-750 MHz. Such devices are frequently used in industry such as in the petrochemical industry.
Noise signals generated on a signal line of the network can cause the detecting means to trigger intermittently the terminator circuit causing the terminator circuit automatically to terminate the network at locations where termination is not required. This can result in instability of the network rendering the network unreliable in certain situations. It is to be understood that failure of an industrial communications network can lead to shutdown of manufacturing or other plant relying on the network. This can be costly to an operator of the plant.
A further known problem with this sensing resistor-based method is that the auto-termination circuit is triggered in response to current flow through the sensing resistor. Current flow may be due to a true load or to a circuit fault such as a cable fault or electronic module fault. It is to be understood that this method is unable to detect the presence of a short circuit fault in the network. Short circuit faults can result in failure of an entire network and even damage to devices coupled to the network.